Retinoscope assembly with scale

ABSTRACT

Techniques and retinoscopic apparatus for measuring or determining a patient&#39;s optical error are disclosed. The techniques include overrefraction and can be performed from a fixed position, avoiding the practitioner&#39;s need to move back and forth relative to the patient&#39;s eye. Equipment associated with the apparatus is adapted to record the location of the retinoscope slide (relative to its upper or lower position) during the examination to provide information concerning the optical error present in the patient&#39;s eye. Other associated equipment can include discs of spherical lenses and devices for limiting relative movement of components of the retinoscopic apparatus. Alternatively, the apparatus may include a scale graduated in suitable units for determining the patient&#39;s optical error.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No. 08/184,828 (to issue as U.S. Pat. No. 5,500,698), filed Jan. 21, 1994, entitled "Optical Instruments Such as Retinoscopes Which May Incorporate Multiple Lens Assemblies and Methods of Calibrating Such Instruments," which is a continuation in part of application Ser. No. 08/101,402 (now U.S. Pat. No. 5,430,508), filed Aug. 3, 1993, entitled "Refraction Techniques Using Modified Streak Retinoscope Assembly," both of which applications are incorporated herein in their entireties by this reference.

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for measuring or determining optical errors in the eyes of humans (or animals) and more particularly to techniques for doing so using a modified streak retinoscope.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,597,051 to Copeland, incorporated herein in its entirety by this reference, illustrates an exemplary streak retinoscope assembly. One such commercial assembly, the "Optec 360" retinoscope 10 of FIG. 1, includes a thumb slide 12 adapted to move relative to the retinoscope shaft. As shown in FIG. 2A, advancing slide 12 to its upper position causes the light rays emanating from the retinoscope to be approximately parallel. FIG. 2B, by contrast, illustrates the convergence of the light rays that occurs when slide 12 is moved to its lower position.

Current refraction techniques use the streak retinoscope for neutralizing optical errors. Such techniques are described in, for example, Videotape No. 5063 of the American Academy of Ophthalmology's Continuing Ophthalmic Video Education series, entitled "Retinoscopy: Plus Cylinder Technique," and require use of a phoropter, trial frames, or additional lenses. According to these techniques, the slide of the retinoscope remains in the upper position throughout the retinoscopic process. Only if greater than one diopter of astigmatic error is present do these techniques suggest "enhancing" the patient's cylinder power by lowering the retinoscope slide. By contrast, "enhancing" the streak to estimate the sphere power does not occur.

After the practitioner neutralizes the patient's error by changing the phoropter or trial frame lenses, the values of those lenses are consulted to determine the patient's ocular correction. These refraction techniques essentially use the phoropter or trial frame lenses to make the light rays emanating from the retinoscope conjugate to the patient's fundus. (Although these rays are generally assumed to be parallel for these techniques, my recent empirical studies suggest that they are not for commercially-available retinoscopes.) Doing so in turn causes the rays backscattered by the patient's fundus to be conjugate to the practitioner's eye, permitting neutralization of optical error at a specified working distance.

According to Videotape No. 5063, prior retinoscopic estimating techniques were complex and thus rarely learned or used by the average practitioner. Such "two-handed" techniques require the practitioner to rotate a collar or sleeve on the retinoscope while simultaneously moving the slide up and down, effectively creating "spiral" movement of the slide and sleeve. To perform these techniques, moreover, the practitioner must move back and forth relative to the patient, thereby alternately approaching and receding from the eye under examination. The patient's optical error is then typically estimated based on the width of the focused streak of light emanating from the retinoscope as seen by the practitioner on the patient's pupil. Following "straddling" and other movements of the streak, the patient's cylindrical error axis can ultimately be estimated by comparing the longitudinal axis of the streak to a scale on the phoropter. Moreover, as discussed on page 21 of Dr. J. C. Copeland's manual for "Steak Retinoscopy" printed by Optec, Inc. (which manual is incorporated herein in its entirety by this reference), these prior estimating techniques were "best . . . perform[ed] . . . on the naked eye" and thus did not involve overrefraction of existing prescription lenses.

SUMMARY OF THE INVENTION

The present invention provides retinoscopic techniques for measuring or determining a patient's optical (sphere and cylinder) error. Unlike prior methods, the present techniques can be performed from a fixed position, avoiding the practitioner's need to move back and forth relative to the patient's eye. They also need not involve straddling and can be performed whether or not the patient is wearing existing prescription glasses.

During the examination, equipment connected to the modified streak retinoscope of the invention senses the position of the slide relative to some nominal location (e.g. its upper or lower position), providing information concerning the optical error present in the patient's eye. This information can be used to calculate the resulting power and cylinder axis of appropriate corrective lenses. By contrast with prior retinoscopic techniques, the present invention uses this positional displacement of the lamp filament from the fixed condensing lens within the retinoscope to permit the rays emanating from the device to be conjugate to the patient's fundus.

The Optec 360 retinoscope 10 of FIG. 1 contains a +20.00D condensing lens 14 and a bi-pin lamp 16. When slide 12 is in its upper position (FIG. 2A), the filament of lamp 16 is approximately five centimeters from condensing lens 14. The rays emanating from the filament pass through lens 14 as essentially parallel, therefore, thereby focusing those rays at infinity. Displacing slide 12 from its upper position converges the emanating rays relative to condensing lens 14 according to the formula:

    P=D+(d×D×D)-W

where

P=total vergence power of retinoscopic light rays at the patient's pupil (in diopters)

D=vergence power of emerging retinoscopic light rays (in diopters)

d=distance from the patient's pupil at which retinoscopy is performed (the "working distance") (in meters)

W=working distance (in diopters)

(Equation 1)

In the slide's lower position on the Optec 360 retinoscope 10 (FIG. 2B), the lamp filament is approximately 6.6 centimeters from condensing lens 14. Accordingly, displacing slide 12 of retinoscope 10 can generate dioptric power ranges as shown below for various (exemplary) working distances:

    ______________________________________                                         Working Distance                                                                              Generated Powers                                                (centimeters)  (diopters)                                                      ______________________________________                                         0.0             0.00 to +4.83                                                  2.54           -39.40 to -33.98                                                10.0           -10.00 to +2.83                                                 20.0           -5.00 to +4.50                                                  25.0           -4.00 to +6.67                                                  33.0           -3.00 to +9.50                                                  50.0            -2.00 to +12.59                                                66.0            -1.50 to +18.80                                                100.0           -1.00 to +26.00                                                ______________________________________                                    

By measuring the slide's displacement from, e.g., its upper position, therefore, information concerning the optical power generated using the retinoscope can be obtained at various times during the examination (including when the streak fills the pupil). Embodiments of the modified streak retinoscope of the present invention use a momentary switch and potentiometer connected to the retinoscope to convert this slide displacement quantity into an electrical resistance. This resistance can in turn be measured and used to calculate the optical power necessary to correct the patient's error. The calculation can be performed electronically by a computer adapted to receive the resistance value, for example, permitting the computer to determine the optical correction needed for an overrefracted patient merely by appropriately combining the resistance with the patient's current prescription.

An additional embodiment of the invention provides a scale on the retinoscope graduated in diopters (or portions thereof or of other appropriate units). Such scale permits the practitioner to determine immediately the optical error of the patient merely by comparing the slide displacement with the corresponding value of the scale. It also avoids the need to compute the optical correction electronically for each patient and to connect the retinoscope to a computer or similar equipment. The scale may be affixed or connected to the retinoscope using any suitable means or may be integrally molded as part of or otherwise formed with the retinoscope itself.

Other embodiments of the apparatus of the present invention can incorporate lens discs or carriers permitting substitution of other power lenses for the +20.00D condensing lens included in the Optec 360 retinoscope 10 of FIG. 1. Alternatively, the powers of these additional lenses can be combined with that of the condensing lens, or the slide displacement can be increased, to enhance the dioptric range of the device. Because the operating principles of some commercial retinoscopes are opposite those of the Optec 360, yet other embodiments of the invention function exactly opposite the manner described above. Further embodiments of the invention include means for limiting the movement of the lamp filament relative to the condensing lens to accommodate, for example, manufacturing variances in existing lamps and lenses.

It is therefore an object of the present invention to provide refraction techniques using a streak retinoscope.

It is another object of the present invention to provide refraction techniques that can be performed by a practitioner from a fixed position relative to a patient.

It is also an object of the present invention to provide overrefraction techniques using a streak retinoscope.

It is a further object of the present invention to provide equipment connected to a streak retinoscope that senses the position of the retinoscope slide relative to some nominal location (e.g. its upper or lower position).

It is yet another object of the present invention to provide a streak retinoscope connected to a momentary switch and potentiometer for converting slide displacement into an electrical resistance.

It is an additional object of the present invention to provide electronic means for converting the sleeve displacement value (and patient's existing prescription if overrefraction is performed) into a resulting optical correction for the patient's eye.

It is also an object of the present invention to provide a scale as part of a retinoscope, permitting determination of the patient's optical error through comparing slide displacement with a corresponding value appearing on the scale.

Other objects, features, and advantages of the present invention will become apparent with reference to the remainder of the text and the drawings of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational, partially cut-away view of an Optec 360 retinoscope.

FIGS. 2A-B is a schematic representation of the light rays emanating from the Optec 360 retinoscope of FIG. 1 with the slide in its upper position (FIG. 2A) and lower position (FIG. 2B). (Although FIGS. 2A-B present the generally-accepted understanding of this representation, my recent empirical studies suggest that the rays emanating from the Optec 360 retinoscope with the slide in its upper position are not parallel but rather slightly divergent.)

FIG. 3 is an elevational, partially schematicized view of a modified retinoscope of the present invention.

FIG. 4 is a cross-sectional view of the modified retinoscope taken along lines A--A of FIG. 3.

FIGS. 5A-I are selected "streak images" possibly viewed using the modified streak retinoscope of FIG. 3.

FIGS. 6A-D are additional streak images possibly viewed using the modified streak retinoscope of FIG. 3.

FIGS. 7A-B are, respectively, an elevational, partially schematicized view of an alternate retinoscope of the present invention and its associated lens disc.

FIG. 8 is another view of the retinoscope of FIG. 7A.

FIGS. 9A-B are elevational views of another alternate retinoscope of the present invention.

DETAILED DESCRIPTION

1. Apparatus

As referenced above, FIG. 1 shows an Optec 360 retinoscope 10 having thumb slide 12, condensing lens 14, and lamp 16. Lamp 16 includes a linear filament designed to create the "streak" reflex or reflection seen by the practitioner from the retina of the eye of the patient being examined. Slide 12 moves approximately 1.6 cm along handle 18 so that, in its upper position, the filament of lamp 16 is approximately 5.0 cm from lens 14, which has power of +20.00D. In its lower position, therefore, the filament of lamp 16 is approximately 6.6 cm from lens 14.

In use, light rays emanating from lamp 16 are reflected by mirror 19 approximately 45° into the patient's eye. The practitioner can view the rays backscattered from the patient's retina through a small opening 20 in mirror 19, effectively focusing the backscattered rays into his pupil. In essence, the phoropter or trial frame lenses subsequently placed before the patient are designed to place the patient's eye in focus with the practitioner's eye peering through opening 20.

FIGS. 3-4 illustrate a modified streak retinoscope 22 of the present invention. Retinoscope 22 may be a modified Optec 360 retinoscope 10 (FIG. 1) or any other suitable device having a displaceable slide 24 or some other means for moving a lamp relative to a lens. As shown in FIGS. 3-4, retinoscope 22 includes a potentiometer 28 coupled to slide 24, providing means for converting displacement of the slide 24 along handle 30 into an electrical resistance. This resistance can in turn be measured by ohmmeter 32 connected to potentiometer 28 and used by a computer 36 or other appropriate mechanism to calculate the optical power necessary to correct a patient's error. Merely by appropriately combining the resistance measured by ohmmeter 32 with the patient's current prescription using known equations, computer 36 can rapidly and easily determine the optical correction needed for an overrefracted patient.

FIG. 4 details the coupling between potentiometer 28 and slide 24. Wire 40 directly attaches slide 24 to the recording wire or contact arm 44 of potentiometer 28 so that, as slide 24 is displaced (upward or downward) along handle 30, contact arm 44 moves in a corresponding manner. Accordingly, potentiometer 28 tracks movement of slide 24, indicating its deviation from a nominal position. Those skilled in the art will recognize that other means may be used to sense the position of slide 24 along handle 30, including mechanisms electrically or optically coupled to slide 24 or uncoupled but otherwise capable of providing the necessary information. A momentary switch 48 or other suitable device may be included as part of computer 36 (FIG. 3), retinoscope 22, or elsewhere in the circuitry to provide means for indicating the point at which the practitioner determines that a displacement measurement needs to be recorded.

2. Exemplary Operations

To refract a patient's eye using retinoscope 22, the practitioner need merely assume a (fixed) position a known distance (e.g. 50 cm) from the patient.

A.

For a patient having a solely spherical error between approximately -1.75D and +2.75D, for example, activating retinoscope 22 with slide 24 in its upper position initially provides to the practitioner the streak reflexive image shown in FIG. 5A. Because no astigmatic error is present in this example, neither the width nor intensity of the streak varies as collar or sleeve 50 is rotated ±90°. Lowering slide 24 widens the reflected streak (FIGS. 5B-C) until it fills the patient's pupil as illustrated in FIG. 5D. Again, because the patient has no astigmatic error in this example, rotating sleeve 50 diminishes neither the width nor intensity of the streak (FIG. 5E). At this point momentary switch may be depressed, providing computer 36 information concerning the distance slide 24 has been displaced from its upper position.

B.

For a patient having a (solely) spherical refractive error of -2.00D, a retinoscope 22 located 50 cm from the patient's eye, and slide 24 in its upper position, the practitioner will initially view the images of FIGS. 5D-E. Accordingly, no further refractive effort is needed and the initial position of slide 24 is immediately converted into an electrical resistance and transmitted to computer 36.

C.

For a patient having a myopic (solely) spherical refractive error greater than -2.00D, the images of FIGS. 5D-E are likely not attainable for working distances of 50 cm or greater. To accommodate these larger spherical errors, the practitioner can place a phoropter or trial frame lens of, for example, between -3.00D and -12.00D before the patient (or use the patient's existing prescription lens) and continue lowering slide 24 until the images of FIGS. 5D-E are obtained. Again, at that point the practitioner can simply activate computer 36 to record the displacement information obtained through potentiometer 28. In this case, however, the power of the phoropter, trial frame, or existing prescription lens must be included in the final corrective calculation (either as a separate input to computer 36 or manually after the displacement information is converted into the refractive error). Alternatively, the practitioner can move toward the patient, decreasing the distance between the retinoscope and eye under examination, until he views the images of FIGS. 5D-E. This decreased working distance must be determined and appropriately factored into the value obtained from computer 36, however.

D.

For a patient having a (solely) spherical error greater than +12.59D, the images of FIGS. 5D-E are similarly not likely to be obtained at a working distance of 50 cm. The practitioner in such a case can place a phoropter or trial frame lens of, for example, between +3.00 and +12.00 before the patient (or again use the patient's existing prescriptive lens). With this lens in place, the practitioner can continue lowering slide 24 until the images of FIGS. 5D-E are obtained, at which point he can activate computer 36 to record the displacement information obtained through potentiometer 28. As in connection with the prior example, the power of the phoropter, trial frame, or existing prescription lens must be included in the final corrective calculation.

E.

FIGS. 5F-I and 6A-D illustrate reflections viewed for a patient having a cylindrical error in addition to the spherical errors mentioned in examples A-D. In FIG. 5F-I, the axis of the patient's spherical error is 180°, while in FIG. 6A-D the axis is 45°. For the patient having a cylindrical error principally in the 180° meridian, the practitioner determines the spherical error in the same way as discussed above. Upon rotating sleeve 50 by ±90°, however, the image of FIG. 5F is obtained and the angular orientation of the streak (i.e. 180°) is noted or estimated by the practitioner. The practitioner again lowers slide 24 (FIGS. 5G-H) until the streak fills the pupil (FIG. 5I), at which point computer 36 is utilized to record the displacement of the slide 24. The noted cylinder axis can then be included with the measurements to produce a final corrective prescription.

Embodiments of retinoscope 22 can also incorporate lens discs or carriers to permit lenses of other powers to be substituted for or combined with lens 14. For example, including a disc of spherical lenses in +0.50D increments capable of being optically aligned with opening 20 would enhance the practitioners ability to use retinoscope 22 accurately at any working distance from 0-100 cm. Incorporating a distance finder into retinoscope 22 would additionally permit electronic measurement of the working distance for input into computer 36, while electrically or otherwise coupling the lens disc to the computer would allow direct input of the added spherical power into the computer 36 for use in later calculations.

Other embodiments of retinoscope 22 function opposite the manner described earlier, recording, for example, the distance slide 24 is displaced from its lower position. These embodiments are designed to accommodate the operating principles utilized in some commercial retinoscopes, in which the light rays from lamp 16 are either focused at infinity (or slightly divergent according to my recent empirical studies) when slide 12 is completed lowered. Yet other embodiments of retinoscope 22 contemplate permitting slide 24 to move more than 1.6 cm, providing a greater range of dioptric powers available for refraction.

FIGS. 7A-B and 8 illustrate an embodiment of retinoscope 22 adapted as described above to incorporate a disc 100 of spherical lenses 104. Disc 100 is designed to be rotated by the practitioner as needed to align a particular lens 104 with opening 20. Use of disc 100 increases the flexibility of retinoscope 22, permitting, for example, a presbyopic practitioner to view a clearer pupillary image. While maintaining a fixed retinoscopic working distance, disc 100 can also be rotated to provide more or less exact spherical correction. Exemplary lenses 104 for disc 100 may have spherical powers between 0.00 and +5.00D (in 0.25 or 0.50D increments).

Also shown in FIGS. 7A and 8 is knob 108, which may be a set screw or other adjustable device suitable for limiting movement of the filament of lamp 16 relative to lens 14. Empirical evidence suggests that the distance between lens 14 and the filament of lamp 16 when slide 12 is in the upper position differs significantly between versions of existing commercial retinoscopes. Using knob 108 to limit upward travel of the filament of lamp 16 permits retinoscope 22 to be calibrated to neutralize not only this discrepancy, but other manufacturing and assembly variances (including those in lens 14) as well.

Calibration of retinoscope 22 can easily be accomplished by clamping it, for example, 50 cm from a -2.00D (myopic) schematic eye. The position of slide 12 or 24 can then be adjusted until the streak fills the pupil of the schematic eye and knob 108 set to preclude further upward travel of the filament of lamp 16. Similar calibration can occur whenever lamp 16 is replaced. Although the embodiment of retinoscope 22 shown in FIGS. 7A-B and 8 may be coupled directly or indirectly to computer 36, it need not be and is useful manually in improving existing retinoscopic techniques. Manual versions of retinoscope 22 could also be adapted to include an infrared or other distance finder for measuring and displaying the resulting working distance.

FIG. 9A-B detail embodiments of retinoscope 22 incorporating scale 120 (FIG. 9A) or (hyperopic) scale 124 (FIG. 9B). Also shown in FIG. 9A-B is indicator 130, which when aligned with a gradation of scale 120 or 124 permits determination (or provides information from which to calculate) a patient's optical error. Indicator 130 may be formed (as, e.g., a groove) in or otherwise associated with collar 132, which in turn is coupled to slide 24. In use, therefore, indicator 130 moves relative to scale 120 or 124 as slide 24 is displaced along handle 30.

Scale 120 is designed primarily for use in determining low myopic and hyperopic errors. It accordingly may be graduated in 0.25D or larger increments if desired and extend from -1.75D to +1.75D as illustrated in FIG. 9A. Scale 124, by contrast, has greater utility in estimating larger hyperopic errors between approximately +1.00D and +3.00D and, in FIG. 9B, is so marked. Those skilled in the art will recognize that other gradations and dioptric ranges may be used for scales 120 and 124 if necessary or desired. Scale 120 contemplates use of conventional endpoints during retinoscopy, furthermore, while enhancements phenomenon endpoints may alternatively be used in connection with retinoscopic methods employing scale 124.

Each of scales 120 and 124 is derived for a specified retinoscopic working distance and may be computer generated according to Equation 1 (printed earlier) and the following Equation 2:

    D=B-(100/(F+U))

where

D=vergence power of emerging retinoscopic light rays (in diopters)

B=back vertex power of the biconvex lens of the retinoscope (in diopters)

F=distance of the lamp filament from the back surface of the biconvex lens within the retinoscope when the slide is displaced from the uppermost position (in centimeters)

U=displacement of the slide from the uppermost position (in centimeters)

For a value of U equalling zero, D equals the negative of the sphere required to focus the diverging light rays of retinoscope 22 onto a wall at twenty feet. Sample values obtained from Equations 1 and 2 appear below:

    ______________________________________                                         (Chart 1)                                                                      Focus Distance                                                                               Retinoscopic Distance                                            (cm/in)       (diopters)                                                       ______________________________________                                         127/50.0      +1.25                                                            106/41.4      +1.50                                                            90/35.4       +1.75                                                            80/31.5       +2.00                                                            70/27.5       +2.25                                                            67/26.4       +2.50                                                            64/25.2       +2.75                                                            54/21.3       +3.00                                                            ______________________________________                                    

To retrofit an existing retinoscope with scale 120 or 124, a practitioner may perform a regular manifest refraction for distance vision using conventional refraction techniques. Thereafter, the practitioner may assume his or her "regular" retinoscopic working distance and lower slide 24 until the light streak emanating from the retinoscope fills the patient's pupil. Slide 24 should then be clamped in order to calibrate the retinoscope and to measure the practitioner's retinoscopic distance.

To calibrate the retinoscope, indicator 130 is aligned with the "0.00D" gradation of scale 120 or the "+2.00D" marking of scale 124. To measure his or her working distance, the practitioner may approach a wall (or analogous structure) until the projected image of the retinoscopic beam is sharply focused onto the wall. The focus distance from the midpoint of the retinoscope to the wall is then measured. The retinoscopic distance may thereafter be determined from Chart 1 (which is derived from Equations 1 and 2) and recorded and slide 24 unclamped. Chart 1 is also utilized to recalibrate the retinoscope whenever the retinoscopic bulb is changed.

A practitioner may determine the optical error of appropriate patient's by using the techniques of retinoscopy described herein and merely comparing the position of indicator 130 relative to scale 120 or 124. If the patient's optical error exceeds the boundaries of the scale, lenses of additional power may be held before the patient's eye until retinoscopy can be completed. In such case the error may be calculated simply by adding the additional power of the lenses to the final value read from the scale.

A new technique of retinoscopy, which I call "converging infinity retinoscopy," can be performed with retinoscope 22 as illustrated in FIGS. 3, 7A, and 9A. Converging infinity retinoscopy occurs when the reflected retinoscopic rays from the patient's retina are focused at infinity. To perform this retinoscopy, the convergence or focal point of the light rays must be as stated in Chart 1. With retinoscope 22 of FIG. 3, for example, slide 24 is moved until computer 36 indicates "0.00D." Likewise, for retinoscope 22 of FIG. 9A, indicator 130 is placed at "0.00D" on scale 120. To perform converging infinity retinoscopy with retinoscope 22 of FIG. 7A, slide 24 is fixed in position using knob 108 after performing the calibration technique described above.

Advantages of converging infinity retinoscopy (as compared to conventional retinoscopy) include:

eliminating divergences of retinoscopes and bulbs which give the impression that the patient is accommodating

endpoints which are clearer, brighter, and of better optical qualities

having a relative speed of the pupillary reflex that is the same as the intercept rather than of infinite speed as in conventional retinoscopy

allowing patient's to read the Snellen chart while the retinoscopist is evaluating the retinoscopic endpoints, since no fogging is required as in conventional retinoscopy

eliminating cycloplegics except for non-cooperating patient's

being easier to teach since the retinoscopic endpoints are more consistent with the manifest refraction; and

allowing performance of any retinoscopic estimation technique or conventional retinoscopy when desired.

The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of the present invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of the invention. 

I claim:
 1. A method of determining the optical error of an eye of a patient comprising:a. providing a retinoscope having:i. a housing; ii. a lens positioned within the housing; iii. a light source positioned within the housing so as to be moveable relative to the lens; iv. a slide displaceable along the handle to move the light source relative to the lens; and v. a scale attached to or formed as part of the housing; b. displacing the slide along the handle; and c. comparing the displacement of the slide with a gradation of the scale to determine the optical error of the eye of the patient.
 2. A retinoscope comprising:a. a housing; b. a lens positioned within the housing; c. a light source positioned within the housing so as to be moveable relative to the lens; d. a slide displaceable along the handle to move the light source relative to the lens; and e. a scale attached to or formed as part of the housing. 